Investigators in Project 2 will focus on evaluation of B cell immune responses against gp41-MPER antigens being delivered by different platforms. The specific aims are: Aim 1. To compare B cell responses to monovalent vs. multivalent gp41 MPER antigens. Using multivalent antigen delivery platforms developed in Aims 1 and 2, we will test our hypothesis that multivalent antigens are capable of inducing antibody responses that are superior to monovalent antigens, qualitatively and quantitatively. We will also test a second hypothesis that it is possible to preferentially elicit stronger antibody responses towards a particular epitope (e.g. 2F5 or 4E10 epitopes) by immunizing with multiple variants of the same antigen where the only conserved element amongst all the variants is the target epitope. Aim 2. To enhance B cell immune responses by co-presentation of costimulatory molecules. Our goal is to enhance antibody responses against multivalent antigens using costimulatory molecules. In this regard, we will test our hypothesis that antibody responses against antigens will be stimulated to a greater extent when costimulatory molecules are physically linked to antigens (as they would be in our delivery platforms), compared to when then are presented as free molecules. We will evaluate stimulatory effects of CDI54 and BLyS. Aim 3. To compare the ability of different gp41 immunogens to elicit TFH and GC responses in vivo. Using the gp41 MPER multimers coupled with the different delivery platforms described in Project 1 (GNP or DNA based arrays) and with the co-stimuli described in Aim 2 of this Project, our goal will be to assess the in vivo CD4+ T helper cell and B cell response after immunization. Specifically, we will challenge mice with each of the engineered gp41 vaccines, and measure the T follicular helper (TFH) cell and germinal center (GC) response after a single exposure, and the memory B cell response after single or multiple exposures These findings will be correlated with anti-gp41 antibody responses measured in Aims 1 and 2.